In general, this invention relates to the field of treating and preventing Shiga toxin associated diseases.
In the United States, Shiga toxin (Stx)-producing Escherichia coli (STEC) account for about 110,000 infections per year. Enterohemorrhagic E. coli (EHEC), most notably the serotype O157:H7, is a subset of STEC that is noted for producing Stx mediated disease. A possible complication from an infection with a Stx-producing organism is the hemolytic uremic syndrome (HUS), which is characterized by hemolytic anemia, thrombic thrombocytopenia, and renal failure. There is approximately a 5-10% fatality rate for those with HUS and survivors may have lasting kidney damage. Currently there are no FDA approved therapies or vaccines to combat or prevent illness from a Stx-mediated disease, but several promising options for the future include: a humanized monoclonal antibody that binds to and neutralizes Stx2 and a chimeric StxA2/StxB1 toxoid that elicits a neutralizing response and provides protection against a lethal challenge of Stx1 or Stx2 or Stx1 and Stx2.
There are essentially two main types of Stxs: Stx/Stx1 and Stx2. Stx is produced from Shigella dysenteriae type 1, while Stx1 and Stx2 are produced from Escherichia coli. Stx and Stx1 are virtually identical, with only one amino acid difference in the A subunit. The mature A and B subunits of Stx1 and Stx2 have 68 and 73% similarity, respectively. Despite the amino acid sequence differences, the crystal structures of Stx and Stx2 are remarkably similar (FIG. 1). These toxins can be differentiated by polyclonal antisera: polyclonal antisera raised against Stx1 does not neutralize Stx2 and vice-versa. Variants of Stx1 and Stx2 exist and include Stx1c, Stx1 d, Stx2c, Stx2d, Stx2d-activatable (Stx2-act.), Stx2e, and Stx2f.
Shiga toxins are complex holotoxins with an AB5 structure. The active domain (A), contains an N-glycosidase that depurinates the 28S rRNA of the 60S ribosomal subunit, which stops proteins synthesis and eventually leads to cell death. The A subunit is ˜32 kDa and is proteolytically cleaved by trypsin or furin into a ˜28 kDa A1 subunit and a ˜5 kDa A2 peptide which are connected through a single disulphide bond. The A1 subunit contains the active domain, and the A2 peptide non-covalently tethers the active domain to the binding domain. The binding domain (B) consists of five identical ˜7.7 kDa monomers that form a pentamer through which the C-terminus of the A2 peptide traverses. Each of the B subunit monomers has two cysteine residues that form a disulphide bond within each monomer (FIG. 2). The B pentamer binds the eukaryotic receptor globotriaosyl ceramide (Gb3) (or Gb4 as is the case for Stx2e).
Despite this knowledge about the results of exposure to these toxins, currently there is no known cure or vaccine for HUS. The use of antibiotics may exacerbate the situation by increasing toxin release from bacteria. Thus, there is a need for a compound to prevent or to treat the complications of EHEC produced by Shiga toxin. Such a compound could be used to treat infected subjects and decrease the systemic effects of toxin on the CNS, blood, and kidneys. In addition, if the toxin could be neutralized, antibiotics could be safely given to kill the bacteria in the GI tract. Such a compound could also be used to prevent infectious complications, by treating exposed or high risk individuals before they acquire EHEC infection. Such individuals would include children in day care or the elderly in nursing homes, where a case of EHEC diarrhea has been identified. These individuals are at increased risk to develop EHEC, often with severe complications, and spread of EHEC in these environments is not unusual.